Stirling cryocoolers include a motor for driving the compression and expansion cycles. For example, an integral Stirling cryocooler includes a single motor that cranks both a compression piston and a displacer piston held within a cold finger. As the motor spins faster, more heat is removed from a cold end of the cold finger to cool the load. Conversely, as the motor spins more slowly, less heat is removed. Thus, a cryocooler commonly includes a closed-loop feedback control system to control the motor speed and thus the cooling power of the cryocooler.
An alternating current (AC) motor requires a differential drive to provide the desired alternating current for powering the motor. FIG. 1a illustrates a conventional driver circuit 100 for powering a 60 Hz AC cryocooler motor 105. Driver circuits A and B each includes a pair of stacked transistors (not illustrated) so as to form an H-bridge with regard to motor 105. By appropriately cycling the H-bridge transistors on and off responsive to driving sine wave signals 110 and 115, an AC drive current and voltage is provided to motor 105. Sine wave signal 115 is 180 degrees out of phase to sine wave signal 110 so that driver A sources current while driver B sinks current and vice versa.
In driver circuit 100, a buck converter 120 converts a 50 V power supply to drive a 26 V power rail that supplies drivers A and B. But the peak sinusoidal voltage provided by either of driver circuits A and B is several volts lower than 26 V to properly bias the H-bridge transistors. In that regard, each of sinusoidal drive signals 110 and 115 cycles about a DC median voltage such as 13V as shown in FIG. 1b. The peak amplitude about this median voltage is 21 V to provide an extra 5 V between this maximum amplitude and the 26 V power rail to satisfy the appropriate voltage headroom for the switching transistors in driver circuits A and B. Since that peak voltage is 8 volts higher than the 13 V median voltage, the minimum voltage for the drive signals 110 and 115 is 13V−8 V=5V. The result of the extra voltage headroom is excess power that must be wasted as heat since it is not delivered to the motor. For example, if the power supply headroom is 5 V and the peak drive current is 8 amps, the wasted peak power equals 40 watts. Since energy must be conserved, this excess power is simply distributed as heat within driver circuits A and B. Thus, conventional cryocooler drive circuits have poor power efficiency.
Accordingly, there is a need in the art for improved cryocoolers with higher-efficiency cryocooler drive circuits.